Studies ar continuing to define the molecular basis for the interaction of oral bacteria with human polymorphonuclear leukocytes (PMNs) and to examine the consequences of these recognition processes. A Gal/GalNAc reactive lectin associated with the type 2 fimbriae of certain oral actinomyces initiated phagocytosis and killing of the bacteria by PMNs. This process was accompanied by the production of superoxide anion and the release of the contents of the PMN secondary granules. The interaction of a sialic acid reactive lectin or certain strains of oral streptococci evoked a different pattern of PMN responses. This lectin also stimulated superoxide anion production by the PMNs and elicited the release of the contents of secondary granules. However, unlike the actinomyces lectin, the streptococcal lectin did not initiate PMN dependent bactericidal activity although its interaction with complementary PMN receptors did result in ingestion of the bacteria as determined by fluorescence microscopy. The actinomyces and streptococcal lectins also differed in their abilities to stimulate PMN chemiluminescence. This response was initiated by the actinomyces lectin but not by the streptococcal lectin. Thus, the sequence of PMN activation events induced by the two bacterial lectins are not identical. The receptors for the Fc portion of immunoglobulin G (IgG) and the receptors for fragments (C3b/C3bi) of the third component of complement are involved in the interaction of opsonized bacteria with PMNs. Studies utilizing IgGs raised against different structures (cell surface, type 1 fimbriae or type 2 fimbriae) on oral actinomyces demonstrated IgGs against the bacterial surface and type 1 fimbriae initiated bactericidal activity whereas those specific for the type 2 fimbriae were ineffective. Enhancement of Fc receptor mediated bactericidal activity by C3b/C3b; only occurred when these fragments were directed towards the cell surface. These findings demonstrate that PMN mediated host defense against these oral actinomyces is strictly dependent on the localization of IgGs and C3 fragments on certain bacterial structures.